14.04 - screen not locking?

running 14.04.3 with unity. Since some days, occasionally my screen will not blank and lock; although "brightness and lock" is configured to exactly do that after 3 minutes.

I checked for answers here, but couldn't find anything helpful. And no, I really like the blank unity stuff; so I don't want to install x screensaver (and I never had x screensaver installed on this system).